1. Chapter 11 Interfacing C and Assembly Code

2. Learning Objectives
Different methods exist for interfacing C and assembly code:
Calling assembly from C.
Interrupt calling assembly routine.
Intrinsics.
Programming requirements when interfacing code.

3. Introduction
This chapter shows how to interface C and assembly and how to use intrinsics.
As a general rule the code written in C is used for initialisation and for non-critical (in terms of speed or size) code.
Critical code (in terms of speed/size) can be written in assembly or linear assembly.
There are three different ways to interface C and assembly code:
(1) C code call to the assembly function.
(2) An interrupt can call an assembly function.
(3) Call an assembly instruction using intrinsics.

4. Calling Assembly from C
main () {
y = asmFunction (a, b); }
_asmFunction
b b3
The C and assembly functions share the same resources (e.g. registers).
The C and assembly functions may exchange data.
Therefore code interfacing requires a means of handing-off data and control info and some rules of handling shared registers.

5. Calling Assembly from C
main_c.c
int asm_Function (short, short);
short x = 0x4000, y = 0x2000;
int z;
void main (void) {
z = asm_Function (x, y); }
Use “_” underscore in assembly for all variables or functions declared in C.
Labels also need to be global.
asm_Function.c
int asm_Function (short a, short b) {
int y;
y = (a * b) << 1;
return y; }
asm_Function.asm
.global _asm_Function

6. Passing Arguments between C and Assembly
arg1/r_val
arg3
arg5
arg7
arg9
ret addr
arg2
arg4
arg6
arg8
arg10 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
The following registers are used to pass and return variables when calling an assembly routine from C.

7. Passing Arguments between C and Assembly
0x4000
0x2000 4 5 6 7 8 A B
Before assembly call.
After return from assembly call.
0x8000
0x2000 4 5 6 7 8

8. Passing Arguments between C and Assembly
Problem:
The C code will use some or all of the registers.
The assembly code may also require the use of some or all registers.
If nothing is done then on return to the C code some of the values may have been destroyed by the assembly code.

9. Passing Arguments between C and Assembly
Solution:
Both the C code and assembly code are responsible for saving some registers if they need to use them. A B 1 2 3
C code automatically saves these registers 4 5 6 7 8 9
Assembly code must save these registers - responsibility of the programmer 10 11 12 13 14 15

10. Interfacing C and Assembly Examples
Setup code written in C and interfaced with critical code written in assembly can be found in the following chapters:
\Code\Chapter 14 - Finite Impulse Response Filters
\Code\Chapter 16 - Adaptive Filters
Setup code written in C and interfaced with critical code written in linear assembly can be found in the following chapters:
\Code\Chapter 15 - Infinite Impulse Response Filters
\Code\Chapter 17 - Goertzel Algorithm

11. Chapter 11 Interfacing C and Assembly Code - End -